Pounder, Derrick J. FRCPA; Cormack, Lesley BMSc; Broadbent, Elizabeth BMSc; Millar, John MB
From the Centre for Forensic and Legal Medicine, University of Dundee, Scotland, United Kingdom.
Manuscript received August 3, 2009; accepted November 16, 2009.
Reprints: Derrick J. Pounder, FRCPA, Centre for Forensic and Legal Medicine, University of Dundee, Dundee DD1 4HN, Scotland, United Kingdom. E-mail: firstname.lastname@example.org.
A total of 136 stab wounds were made in cartilage with 8 serrated knives and 72 stabs with 4 nonserrated knives. The walls of the stab track were documented by photography, cast with dental impression material, and the casts photographed. Staining the translucent cartilage surface with blue or green food dye improved photography. Serrated blades produced striations on cartilage in all stabbings. Patterns of blade serration beyond the broad categories of coarse and fine were recognizable. The overall pattern of striations was "irregularly regular." The distance between the blade-spine wound end and the first serration striation is a class characteristic of the knife which produced the defect, as are distances to the subsequent serration striations, which become ever close together and eventually merge near the blade-edge wound end. Serrated knives may be ground (scalloped) on either the left side or the right side of the blade and this class characteristic is identifiable from the walls of the wound track, on which the scalloped blade surface produces broad ridges and narrow striation valleys, with a reverse image on the opposing wound wall. A drop point serrated blade consistently produced an additional oblique mark angled from the blade-spine wound end, accurately reflecting the shape of the blade tip, and representing a chatter mark.
Stabbing is a common method of homicide in many jurisdictions, and often the knife is a weapon of opportunity in the home.1 In a domestic setting, serrated knives are increasingly common, as is their use in homicides. Standard forensic texts state that stab wounds caused by serrated blades are not generally distinguishable from those caused by nonserrated blades.2 An important exception, recorded in the literature but not widely appreciated, is that stab wounds from serrated knives involving costal cartilages leave striations on the cut surface of the cartilage from the serration points on the blade edge. The renowned German forensic expert Wolfgang Bonte (1939-2000) first drew attention to these tool marks on cartilage in the English language literature,3 having already published on the phenomenon in the German literature.4 Rao and Hart5 later reported a case in which grossly visible striae were observed on cut costal cartilages in a homicidal stabbing in which a serrated knife had been used. The literature contains few follow-up studies over the past 25 years,6-9 a fact adversely commented upon in an appeal against a murder conviction in Florida,10-12 which itself initiated vigorous debate.13,14
In light of the increased availability of serrated knives, the high frequency of anterior thorax wounds in homicidal stabbings,15 and the potential value of the examination of severed costal cartilage for the purpose of including or excluding a putative weapon, we undertook a study of experimental stab wounds in porcine and bovine costal cartilage using a variety of serrated blades to assess the potential utility of the technique.
MATERIALS AND METHODS
Serrated knives were purchased in department stores and their characteristics documented. Standard terminology with respect to knife blade anatomy has been used16: the edge of the blade refers to the working part or cutting surface; the spine of the blade refers to the top of the blade opposite the edge, and is unsharpened in the single-edged blades used in these experiments; the tip of the blade refers to the forward quarter of the blade and includes the point, which is the very end part of the blade; the right side of the blade is the side towards the right of a person holding the knife by the handle, tip uppermost, with the edge facing away from the holder's body, and similarly the left side of the blade is on the left side of that person. (When displayed knives are traditionally positioned with the handle to the right side of the viewer, the blade to the left side of the viewer, with the edge below and the spine above, so that the left side of the blade faces the viewer). The term "back of the blade" has not been used since this may refer to either the spine or the right side of the blade so giving rise to confusion. Serrations17 are introduced during the final grinding process by removing scallops of metal from the blade edge on one side (either the right or the left) leaving the other side flat, so that serrated blades may be either right side ground (scalloped) or left side ground (scalloped).
Porcine and bovine rib cages were sourced from a commercial butcher and lengths of rib cartilage were dissected away from the sternum and ribs. The cartilage was tethered at both ends with string tied to nails at the corners of a wooden chopping board with the cartilage sitting upon a 500 g block of modeling clay, to allow for penetration of the blade through the cartilage. The stabbing was achieved slowly with the knife at 90° to the cartilage and the knife then withdrawn. The section of cartilage with the stab wound was trimmed to a smaller size and the stab wound opened by inserting a scalpel blade back first into the wound and rotating the scalpel so as to extend the ends of the stab canal to the margins of the cartilage without damaging the stab track. Stabs were performed and stab wounds dissected in a systematic way so that the right wall of the stab wound corresponding to the right side of the knife blade and the left wall of the stab wound corresponding to the left side of the knife blade were always identified. The exposed stab wound surface was rinsed with water and blotted dry with paper towel. The cut surfaces were examined by naked eye and then photographed using markedly oblique lighting to accentuate any striations. A range of colors (red, blue, green, yellow, and black) of food dye were used to stain the cartilage in an attempt to reduce specimen translucency and reflectance which made photo-documentation difficult. After photo-documentation the cut surfaces were cast using Provil Novo Medium C.D.2 fast set vinyl polysiloxane dental impression material (Heraeus Kultzer, South Bend, IN). The casts were photographed.
RESULTS AND DISCUSSION
Stab wounds made using a drop point coarsely serrated blade (32 stabs), 3 different straight spine coarsely serrated blades (60 stabs), a straight spine finely serrated blade (12 stabs), a straight spine mixed coarse and finely serrated blade (12 stabs), a drop point finely serrated blade (20 stabs), and a clip point blade with coarse serrations, produced prominent striations on the cartilage in all cases. Stabs with 3 different drop point nonserrated blades (60 stabs) and a straight spine nonserrated blade (12 stabs) produced no gross striations of the type produced by serrated blades, although much finer striations corresponding to blade edge imperfections could sometimes be seen. Photography of striation marks on the cut surface of the cartilage was difficult due to translucency and reflections.5 Prolonged exposure to the heat of photographic lighting dried the specimen. Staining the cartilage surface with a blue food dye improved the quality of photographs, as did the green dye but to a lesser extent. There was no substantial improvement using red, yellow, or black food dyes. Photography of the opaque dental casting material was straightforward and striation marks were more readily recorded in photographs of casts.
In all of the stab wounds, using a variety of serrated knives it was possible to identify by naked eye examination striations on the cut surface of the cartilage corresponding with the serration points of the blade. Stabs produced by coarsely serrated blades were easily distinguished from those produced by finely serrated blades. The pattern of serrations on a standard coarsely serrated knife blade is characterized by a gap between the point of the blade and the first serration point, with subsequent serration points being evenly spaced along the length of the blade. With the knife blade passing vertically through the cartilage this serration pattern on the blade leaves a significant gap between the wound end corresponding to the blade spine and the first striation representing the first serration point. Subsequent striations on the cartilage, corresponding to serration points further along the blade, are separated by decreasing distances between each other, corresponding with the projected alignment of the serrations relative to the width of the blade (Fig. 1). Toward the other end of the wound, corresponding to the cutting edge of the blade, the striation marks are so close together or overlaid that they are not individually recognizable. The pattern could be characterized as "irregularly regular." It is this overall pattern which is characteristic of a serrated blade.
With finely serrated blades the individual fine striations are visible and grouped, as are the serrations on the blade, with sparing between the groupings. The overall pattern produced is the same as with coarsely serrated blades except that the single coarse striations are replaced by groups of fine striations which become ever narrower with an ever reducing distance between the groupings ie, the same "irregularly regular" pattern. The pattern of fine serrations on a blade is an identifiable class characteristic as a result of between-blade variability in the number of striations per group, the extent of sparing between striation groups and the presence or absence of interspersed coarse striations.
Within the overall pattern of striations there is a variability which reflects class characteristics of the blade. The distance between the end of the wound representing the spine of the blade and the first serration striation mark is a class characteristic, as are the distances of all of the subsequent serration striations (or groups of striations from finely serrated blades) from the blade-spine wound end. The 2 factors determining these latter distances would appear to be the distance between the serration points on the blade and the shape of the blade. Since both of these factors are unknown if the blade is not available, it would seem impossible to reliably deduce either from the pattern of serrations. It may be possible, at least in theory, for two quite different blades, having different blade shapes and different separations of serration points, to produce the same or a closely similar pattern of striation marks. Therefore, some caution should be exercised in weighing the significance of a serration pattern which appears identical to a putative weapon until further experience and data is acquired.
With coarsely serrated blades, the striations observed correspond with the prominent tips of the serrations on the blade so that the space between them corresponds with the scalloped edge of the serration. If the scalloping is on the left side of the blade then the scallops create broad ridges on the left wall of the wound with narrow striation valleys between representing the serration points. By contrast the right wall of the same wound track shows mirror-image broad valleys with narrow striation ridges between. In this way an examination of the 2 walls of the wound allows a determination of whether the serrated blade which produced it was left side scalloped (ground) or right side scalloped (ground). When making this assessment from casts rather than the original wound it is important to remember that the cast is a reverse image of the original, so that the cast of the right wall of the wound has the same appearance as the original left wall of the wound. In both the original specimen and casts it is easier to observe the striations when they are present as ridges rather than valleys, because they stand out more sharply in oblique lighting (Figs. 2 and 3).
In one set of experiments, in all of 20 of stab wounds using a drop point coarsely serrated blade, an oblique mark was observed passing from the wound edge corresponding to the spine of the blade upwards towards the surface of the wound in the cartilage. The mark was present but faint in 20%; present, fully formed but not readily obvious to an untrained eye in 50%; and clearly visible even to an untrained eye in 30% (Fig. 4). By contrast for a straight spine coarsely serrated blade the mark was seen in only 1 of 20 stabs, and then it was fully formed but not obvious to an untrained eye (Fig. 5). A similar mark was not observed in 20 stab wounds made with a drop point nonserrated blade.
Comparison of this mark with the knives indicated that it represents the blade edge at the tip. The V-shape between the blade-spine wound end and this mark reflects accurately the shape of the tip of the blade (Fig. 6). The location of this unusual mark suggests that it was produced when tissue was initially trapped in the concavity of the first scallop, causing the blade to jar slightly. If this proposed mechanism of formation is correct then it would be appropriate to characterize the mark as a "blade tip chatter mark." The greater prominence and frequency of this chatter mark in stabs from the drop point serrated blade may be the result of either the more prominent serrations of that blade and/or the drop point shape which increases the initial lateral movement of the blade during the stabbing. It remains to be seen whether this is merely a feature of the experimental model or is observable in case material.
In summary the class characteristics of a serrated blade which might be determined from striation marks in cartilage include (a) the overall pattern of coarse and/or fine serrations, (b) the distance between the spine of the blade and the first serration point, (c) the distances between the spine of the blade and the subsequent serration points, (d) whether the blade was right side or left side ground (scalloped), and (e) the shape of the tip of the blade if a chatter mark is present. We have not addressed in this study the issue of finer striations produced by imperfections in blades, both serrated and nonserrated, which may be individualizing characteristics and so provide further valuable tool mark information.
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